Release 2 (9.2)
Part Number A96654-01
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| Oracle9i JDBC Developer's Guide and Reference Release 2 (9.2) Part Number A96654-01 |
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This chapter covers the most basic steps taken in any JDBC application. It also describes additional basic features of Java and JDBC supported by the Oracle JDBC drivers.
The following topics are discussed:
This section describes how to get up and running with the Oracle JDBC drivers. When using the Oracle JDBC drivers, you must include certain driver-specific information in your programs. This section describes, in the form of a tutorial, where and how to add the information. The tutorial guides you through creating code to connect to and query a database from the client.
To connect to and query a database from the client, you must provide code for these tasks:
You must supply Oracle driver-specific information for the first three tasks, which allow your program to use the JDBC API to access a database. For the other tasks, you can use standard JDBC Java code as you would for any Java application.
Regardless of which Oracle JDBC driver you use, include the following import statements at the beginning of your program (java.math only if needed):
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for standard JDBC packages |
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for |
Import the following Oracle packages when you want to access the extended functionality provided by the Oracle drivers. However, they are not required for the example presented in this section:
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for Oracle extensions to JDBC |
For an overview of the Oracle extensions to the JDBC standard, see Chapter 6, "Overview of Oracle Extensions".
You must provide the code to register your installed driver with your program. You do this with the static registerDriver() method of the JDBC DriverManager class. This class provides a basic service for managing a set of JDBC drivers.
Because you are using one of Oracle's JDBC drivers, you declare a specific driver name string to registerDriver(). You register the driver only once in your Java application.
DriverManager.registerDriver (new oracle.jdbc.OracleDriver());
Open a connection to the database with the static getConnection() method of the JDBC DriverManager class. This method returns an object of the JDBC Connection class that needs as input a user name, password, connect string that identifies the JDBC driver to use, and the name of the database to which you want to connect.
Connecting to a database is a step where you must enter Oracle JDBC driver-specific information in the getConnection() method. If you are not familiar with this method, continue reading the "Understanding the Forms of getConnection()" section below.
If you are already familiar with the getConnection() method, you can skip ahead to either of these sections, depending on the driver you installed:
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Notes:
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The DriverManager class getConnection() method whose signatures and functionality are described in the following sections:
If you want to specify a database name in the connection, it must be in one of the following formats:
TNSNAMES entry (OCI driver only)For information on how to specify a keyword-value pair or a TNSNAMES entry, see your Oracle Net Services Administrator's Guide.
The following signature takes the URL, user name, and password as separate parameters:
getConnection(StringURL, Stringuser, Stringpassword);
Where the URL is of the form:
jdbc:oracle:<drivertype>:@<database>
The following example connects user scott with password tiger to a database with INSTANCE_NAME orcl through port 1521 of host myhost, using the Thin driver.
Connection conn = DriverManager.getConnection ("jdbc:oracle:thin:@myhost:1521:orcl", "scott", "tiger");
If you want to use the default connection for an OCI driver, specify either:
Connection conn = DriverManager.getConnection ("jdbc:oracle:oci:scott/tiger@");
or:
Connection conn = DriverManager.getConnection ("jdbc:oracle:oci:@", "scott", "tiger");
For all JDBC drivers, you can also specify the database with a Oracle Net keyword-value pair. The Oracle Net keyword-value pair substitutes for the TNSNAMES entry. The following example uses the same parameters as the preceding example, but in the keyword-value format:
Connection conn = DriverManager.getConnection (jdbc:oracle:oci:@MyHostString","scott","tiger");
or:
Connection conn = DriverManager.getConnection ("jdbc:oracle:oci:@(description=(address=(host= myhost) (protocol=tcp)(port=1521))(connect_data=(INSTANCE_NAME=orcl)))", "scott", "tiger");
The following signature takes the URL, user name, and password all as part of a URL parameter:
getConnection(String URL);
Where the URL is of the form:
jdbc:oracle:<drivertype>:<user>/<password>@<database>
The following example connects user scott with password tiger to a database on host myhost using the OCI driver. In this case, however, the URL includes the userid and password, and is the only input parameter.
Connection conn = DriverManager.getConnection ("jdbc:oracle:oci:scott/tiger@myhost);
If you want to connect with the Thin driver, you must specify the port number and SID. For example, if you want to connect to the database on host myhost that has a TCP/IP listener up on port 1521, and the SID (system identifier) is orcl:
Connection conn = DriverManager.getConnection ("jdbc:oracle:thin:scott/tiger@myhost:1521:orcl);
The following signature takes a URL, together with a properties object that specifies user name and password (perhaps among other things):
getConnection(StringURL, Propertiesinfo);
Where the URL is of the form:
jdbc:oracle:<drivertype>:@<database>
In addition to the URL, use an object of the standard Java Properties class as input. For example:
java.util.Properties info = new java.util.Properties(); info.put ("user", "scott"); info.put ("password","tiger"); info.put ("defaultRowPrefetch","15"); getConnection ("jdbc:oracle:oci:@",info);
Table 3-1 lists the connection properties that Oracle JDBC drivers support.
See Table 18-4, "OCI Driver Client Parameters for Encryption and Integrity" and Table 18-5, "Thin Driver Client Parameters for Encryption and Integrity" for descriptions of encryption and integrity drivers.
To specify the role (mode) for sys logon, use the internal_logon connection property. (See Table 3-1, "Connection Properties Recognized by Oracle JDBC Drivers", for a complete description of this connection property.) To logon as sys, set the internal_logon connection property to sysdba or sysoper.
The following example illustrates how to use the internal_logon and sysdba arguments to specify sys logon.
//import packages and register the driver import java.sql.*; import java.math.*; DriverManager.registerDriver (new oracle.jdbc.OracleDriver()); //specify the properties object java.util.Properties info = new java.util.Properties(); info.put ("user", "sys"); info.put ("password", "change_on_install"); info.put ("internal_logon","sysdba"); //specify the connection object Connection conn = DriverManager.getConnection ("jdbc:oracle:thin:@database",info); ...
Some of these properties are for use with Oracle performance extensions. Setting these properties is equivalent to using corresponding methods on the OracleConnection object, as follows:
defaultRowPrefetch property is equivalent to calling setDefaultRowPrefetch().
remarksReporting property is equivalent to calling setRemarksReporting().
defaultBatchValue property is equivalent to calling setDefaultExecuteBatch().
The following example shows how to use the put() method of the java.util.Properties class, in this case to set Oracle performance extension parameters.
//import packages and register the driver import java.sql.*; import java.math.*; DriverManager.registerDriver (new oracle.jdbc.OracleDriver()); //specify the properties object java.util.Properties info = new java.util.Properties(); info.put ("user", "scott"); info.put ("password", "tiger"); info.put ("defaultRowPrefetch","20"); info.put ("defaultBatchValue", "5"); //specify the connection object Connection conn = DriverManager.getConnection ("jdbc:oracle:thin:@database",info); ...
For the JDBC OCI driver, you can specify the database by a TNSNAMES entry. You can find the available TNSNAMES entries listed in the file tnsnames.ora on the client computer from which you are connecting. On Windows NT, this file is located in the [ORACLE_HOME]\NETWORK\ADMIN directory. On UNIX systems, you can find it in the /var/opt/oracle directory.
For example, if you want to connect to the database on host myhost as user scott with password tiger that has a TNSNAMES entry of MyHostString, enter:
Connection conn = DriverManager.getConnection ("jdbc:oracle:oci:@MyHostString", "scott", "tiger");
Note that both the ":" and "@" characters are necessary.
For the JDBC OCI and Thin drivers, you can also specify the database with a Oracle Net keyword-value pair. This is less readable than a TNSNAMES entry but does not depend on the accuracy of the TNSNAMES.ORA file. The Oracle Net keyword-value pair also works with other JDBC drivers.
For example, if you want to connect to the database on host myhost that has a TCP/IP listener up on port 1521, and the SID (system identifier) is orcl, use a statement such as:
Connection conn = DriverManager.getConnection ("jdbc:oracle:oci:@(description=(address=(host= myhost) (protocol=tcp)(port=1521))(connect_data=(INSTANCE_NAME=orcl)))", "scott", "tiger");
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Note: Oracle JDBC does not support login timeouts. Calling the static |
Because you can use the JDBC Thin driver in applets that do not depend on an Oracle client installation, you cannot use a TNSNAMES entry to identify the database to which you want to connect. You have to either:
or:
For example, use this string if you want to connect to the database on host myhost that has a TCP/IP listener on port 1521 for the database SID (system identifier) orcl. You can logon as user scott, with password tiger:
Connection conn = DriverManager.getConnection ("jdbc:oracle:thin:@myhost:1521:orcl", "scott", "tiger");
You can also specify the database with a Oracle Net keyword-value pair. This is less readable than the first version, but also works with the other JDBC drivers.
Connection conn = DriverManager.getConnection ("jdbc:oracle:thin:@(description=(address=(host=myhost) (protocol=tcp)(port=1521))(connect_data=(INSTANCE_NAME=orcl)))", "scott", "tiger");
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Notes: Oracle JDBC does not support login timeouts. Calling the static |
Once you connect to the database and, in the process, create your Connection object, the next step is to create a Statement object. The createStatement() method of your JDBC Connection object returns an object of the JDBC Statement class. To continue the example from the previous section where the Connection object conn was created, here is an example of how to create the Statement object:
Statement stmt = conn.createStatement();
Note that there is nothing Oracle-specific about this statement; it follows standard JDBC syntax.
To query the database, use the executeQuery() method of your Statement object. This method takes a SQL statement as input and returns a JDBC ResultSet object.
To continue the example, once you create the Statement object stmt, the next step is to execute a query that populates a ResultSet object with the contents of the ENAME (employee name) column of a table of employees named EMP:
ResultSet rset = stmt.executeQuery ("SELECT ename FROM emp");
Again, there is nothing Oracle-specific about this statement; it follows standard JDBC syntax.
Once you execute your query, use the next() method of your ResultSet object to iterate through the results. This method steps through the result set row by row, detecting the end of the result set when it is reached.
To pull data out of the result set as you iterate through it, use the appropriate getXXX() methods of the ResultSet object, where XXX corresponds to a Java datatype.
For example, the following code will iterate through the ResultSet object rset from the previous section and will retrieve and print each employee name:
while (rset.next()) System.out.println (rset.getString(1));
Once again, this is standard JDBC syntax. The next() method returns false when it reaches the end of the result set. The employee names are materialized as Java strings.
You must explicitly close the ResultSet and Statement objects after you finish using them. This applies to all ResultSet and Statement objects you create when using the Oracle JDBC drivers. The drivers do not have finalizer methods; cleanup routines are performed by the close() method of the ResultSet and Statement classes. If you do not explicitly close your ResultSet and Statement objects, serious memory leaks could occur. You could also run out of cursors in the database. Closing a result set or statement releases the corresponding cursor in the database.
For example, if your ResultSet object is rset and your Statement object is stmt, close the result set and statement with these lines:
rset.close();stmt.close();
When you close a Statement object that a given Connection object creates, the connection itself remains open.
To write changes to the database, such as for INSERT or UPDATE operations, you will typically create a PreparedStatement object. This allows you to execute a statement with varying sets of input parameters. The prepareStatement() method of your JDBC Connection object allows you to define a statement that takes variable bind parameters, and returns a JDBC PreparedStatement object with your statement definition.
Use setXXX() methods on the PreparedStatement object to bind data into the prepared statement to be sent to the database. The various setXXX() methods are described in "Standard setObject() and Oracle setOracleObject() Methods" and "Other setXXX() Methods".
Note that there is nothing Oracle-specific about the functionality described here; it follows standard JDBC syntax.
The following example shows how to use a prepared statement to execute INSERT operations that add two rows to the EMP table.
// Prepare to insert new names in the EMP table PreparedStatement pstmt = conn.prepareStatement ("insert into EMP (EMPNO, ENAME) values (?, ?)"); // Add LESLIE as employee number 1500 pstmt.setInt (1, 1500); // The first ? is for EMPNO pstmt.setString (2, "LESLIE"); // The second ? is for ENAME // Do the insertion pstmt.execute (); // Add MARSHA as employee number 507 pstmt.setInt (1, 507); // The first ? is for EMPNO pstmt.setString (2, "MARSHA"); // The second ? is for ENAME // Do the insertion pstmt.execute (); // Close the statement pstmt.close();
By default, DML operations (INSERT, UPDATE, DELETE) are committed automatically as soon as they are executed. This is known as auto-commit mode. You can, however, disable auto-commit mode with the following method call on the Connection object:
conn.setAutoCommit(false);
(For further discussion of auto-commit mode and an example of disabling it, see "Disabling Auto-Commit Mode".)
If you disable auto-commit mode, then you must manually commit or roll back changes with the appropriate method call on the Connection object:
conn.commit();
or:
conn.rollback();
A COMMIT or ROLLBACK operation affects all DML statements executed since the last COMMIT or ROLLBACK.
You must close your connection to the database once you finish your work. Use the close() method of the Connection object to do this:
conn.close();
The steps in the preceding sections are illustrated in the following example, which registers an Oracle JDBC Thin driver, connects to the database, creates a Statement object, executes a query, and processes the result set.
Note that the code for creating the Statement object, executing the query, returning and processing the ResultSet object, and closing the statement and connection all follow standard JDBC syntax.
import java.sql.*;
import java.math.*;
import java.io.*;
import java.awt.*;
class JdbcTest {
public static void main (String args []) throws SQLException {
// Load Oracle driver
DriverManager.registerDriver (new oracle.jdbc.OracleDriver());
// Connect to the local database
Connection conn = DriverManager.getConnection
("jdbc:oracle:thin:@myhost:1521:ORCL","scott", "tiger");
// Query the employee names
Statement stmt = conn.createStatement ();
ResultSet rset = stmt.executeQuery ("SELECT ename FROM emp");
// Print the name out
while (rset.next ())
System.out.println (rset.getString (1));
//close the result set, statement, and the connection
rset.close();
stmt.close();
conn.close();
}
}
If you want to adapt the code for the OCI driver, replace the Connection statement with the following:
Connection conn = DriverManager.getConnection ("jdbc:oracle:oci:@MyHostString", "scott", "tiger");
Where MyHostString is an entry in the TNSNAMES.ORA file.
The Oracle JDBC drivers support standard JDBC 1.0 and 2.0 types as well as Oracle-specific BFILE and ROWID datatypes and types of the REF CURSOR category.
This section documents standard and Oracle-specific SQL-Java default type mappings.
For reference, Table 3-2 shows the default mappings between SQL datatypes, JDBC typecodes, standard Java types, and Oracle extended types.
The SQL Datatypes column lists the SQL types that exist in the database.
The JDBC Typecodes column lists data typecodes supported by the JDBC standard and defined in the java.sql.Types class, or by Oracle in the oracle.jdbc.OracleTypes class. For standard typecodes, the codes are identical in these two classes.
The Standard Java Types column lists standard types defined in the Java language.
The Oracle Extension Java Types column lists the oracle.sql.* Java types that correspond to each SQL datatype in the database. These are Oracle extensions that let you retrieve all SQL data in the form of a oracle.sql.* Java type. Mapping SQL datatypes into the oracle.sql datatypes lets you store and retrieve data without losing information. Refer to "Package oracle.sql" for more information on the oracle.sql.* package.
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Note: Under JDK 1.1.x, the Oracle package |
For a list of all the Java datatypes to which you can validly map a SQL datatype, see "Valid SQL-JDBC Datatype Mappings".
See Chapter 6, "Overview of Oracle Extensions", for more information on type mappings. In Chapter 6 you can also find more information on the following:
oracle.sql, oracle.jdbc, and oracle.jdbc2BFILE and ROWID datatypes and user-defined types of the REF CURSOR categoryThis section goes into further detail regarding mappings for NUMBER and user-defined types.
User-defined types such as objects, object references, and collections map by default to weak Java types (such as java.sql.Struct), but alternatively can map to strongly typed custom Java classes. Custom Java classes can implement one of two interfaces:
java.sql.SQLData (for user-defined objects only)oracle.sql.ORAData (primarily for user-defined objects, object references, and collections, but able to map from any SQL type where you want customized processing of any kind)For information about custom Java classes and the SQLData and ORAData interfaces, see "Mapping Oracle Objects" and "Creating and Using Custom Object Classes for Oracle Objects". (Although these sections focus on custom Java classes for user-defined objects, there is some general information about other kinds of custom Java classes as well.)
For the different typecodes that an Oracle NUMBER value can correspond to, call the getter routine that is appropriate for the size of the data for mapping to work properly. For example, call getByte() to get a Java tinyint value, for an item x where -128 < x < 128.
This section covers the following topics:
This section describes how the Oracle JDBC drivers handle Java streams for several datatypes. Data streams allow you to read LONG column data of up to 2 gigabytes. Methods associated with streams let you read the data incrementally.
Oracle JDBC drivers support the manipulation of data streams in either direction between server and client. The drivers support all stream conversions: binary, ASCII, and Unicode. Following is a brief description of each type of stream:
RAW bytes of data. This corresponds to the getBinaryStream() method.getAsciiStream() method.UTF-16 encoding. This corresponds to the getUnicodeStream() method.The methods getBinaryStream(), getAsciiStream(), and getUnicodeStream() return the bytes of data in an InputStream object. These methods are described in greater detail in Chapter 8, "Working with LOBs and BFILEs".
When a query selects one or more LONG or LONG RAW columns, the JDBC driver transfers these columns to the client in streaming mode. After a call to executeQuery() or next(), the data of the LONG column is waiting to be read.
To access the data in a LONG column, you can get the column as a Java InputStream and use the read() method of the InputStream object. As an alternative, you can get the data as a string or byte array, in which case the driver will do the streaming for you.
You can get LONG and LONG RAW data with any of the three stream types. The driver performs conversions for you, depending on the character set of your database and the driver. For more information about globalization support, see "JDBC and Globalization Support".
A call to getBinaryStream() returns RAW data "as-is". A call to getAsciiStream() converts the RAW data to hexadecimal and returns the ASCII representation. A call to getUnicodeStream() converts the RAW data to hexadecimal and returns the Unicode bytes.
For example, if your LONG RAW column contains the bytes 20 21 22, you receive the following bytes:
| LONG RAW | BinaryStream | ASCIIStream | UnicodeStream |
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20 21 22 |
20 21 22 |
'1' '4' '1' '5' '1' '6' |
'1' '4' '1' '5' '1' '6' |
For example, the LONG RAW value 20 is represented in hexadecimal as 14 or "1" "4". In ASCII, 1 is represented by "49" and "4" is represented by "52". In Unicode, a padding of zeros is used to separate individual values. So, the hexadecimal value 14 is represented as 0 "1" 0 "4". The Unicode representation is 0 "49" 0 "52".
When you get LONG data with getAsciiStream(), the drivers assume that the underlying data in the database uses an US7ASCII or WE8ISO8859P1 character set. If the assumption is true, the drivers return bytes corresponding to ASCII characters. If the database is not using an US7ASCII or WE8ISO8859P1 character set, a call to getAsciiStream() returns meaningless information.
When you get LONG data with getUnicodeStream(), you get a stream of Unicode characters in the UTF-16 encoding. This applies to all underlying database character sets that Oracle supports.
When you get LONG data with getBinaryStream(), there are two possible cases:
US7ASCII or WE8ISO8859P1, then a call to getBinaryStream() returns UTF-8. If the client character set is US7ASCII or WE8ISO8859P1, then the call returns a US7ASCII stream of bytes.US7ASCII or WE8ISO8859P1, then a call to getBinaryStream() returns UTF-8. If the server-side character set is US7ASCII or WE8ISO8859P1, then the call returns a US7ASCII stream of bytes.For more information on how the drivers return data based on character set, see "JDBC and Globalization Support".
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Note: Receiving |
Table 3-3 summarizes LONG and LONG RAW data conversions for each stream type.
One of the features of a getXXXStream() method is that it allows you to fetch data incrementally. In contrast, getBytes() fetches all the data in one call. This section contains two examples of getting a stream of binary data. The first version uses the getBinaryStream() method to obtain LONG RAW data; the second version uses the getBytes() method.
This Java example writes the contents of a LONG RAW column to a file on the local file system. In this case, the driver fetches the data incrementally.
The following code creates the table that stores a column of LONG RAW data associated with the name LESLIE:
-- SQL code: create table streamexample (NAME varchar2 (256), GIFDATA long raw); insert into streamexample values ('LESLIE', '00010203040506070809');
The following Java code snippet writes the data from the LESLIE LONG RAW column into a file called leslie.gif:
ResultSet rset = stmt.executeQuery ("select GIFDATA from streamexample where NAME='LESLIE'"); // get first row if (rset.next()) { // Get the GIF data as a stream from Oracle to the client InputStream gif_data = rset.getBinaryStream (1); try { FileOutputStream file = null; file = new FileOutputStream ("leslie.gif"); int chunk; while ((chunk = gif_data.read()) != -1) file.write(chunk); } catch (Exception e) { String err = e.toString(); System.out.println(err); } finally { if file != null() file.close(); } }
In this example the contents of the GIFDATA column are transferred incrementally in chunk-sized pieces between the database and the client. The InputStream object returned by the call to getBinaryStream() reads the data directly from the database connection.
This version of the example gets the content of the GIFDATA column with getBytes() instead of getBinaryStream(). In this case, the driver fetches all the data in one call and stores it in a byte array. The previous code snippet can be rewritten as:
ResultSet rset2 = stmt.executeQuery ("select GIFDATA from streamexample where NAME='LESLIE'"); // get first row if (rset2.next()) { // Get the GIF data as a stream from Oracle to the client byte[] bytes = rset2.getBytes(1); try { FileOutputStream file = null; file = new FileOutputStream ("leslie2.gif"); file.write(bytes); } catch (Exception e) { String err = e.toString(); System.out.println(err); } finally { if file != null() file.close(); } }
Because a LONG RAW column can contain up to 2 gigabytes of data, the getBytes() example will probably use much more memory than the getBinaryStream() example. Use streams if you do not know the maximum size of the data in your LONG or LONG RAW columns.
The JDBC driver automatically streams any LONG and LONG RAW columns. However, there may be situations where you want to avoid data streaming. For example, if you have a very small LONG column, you might want to avoid returning the data incrementally and instead, return the data in one call.
To avoid streaming, use the defineColumnType() method to redefine the type of the LONG column. For example, if you redefine the LONG or LONG RAW column as type VARCHAR or VARBINARY, then the driver will not automatically stream the data.
If you redefine column types with defineColumnType(), you must declare the types of all columns in the query. If you do not, executeQuery() will fail. In addition, you must cast the Statement object to an oracle.jdbc.OracleStatement object.
As an added benefit, using defineColumnType() saves the driver two round trips to the database when executing the query. Without defineColumnType(), the JDBC driver has to request the datatypes of the column types.
Using the example from the previous section, the Statement object stmt is cast to the OracleStatement and the column containing LONG RAW data is redefined to be of the type VARBINARAY. The data is not streamed--instead, it is returned in a byte array.
//cast the statement stmt to an OracleStatement oracle.jdbc.OracleStatement ostmt = (oracle.jdbc.OracleStatement)stmt; //redefine the LONG column at index position 1 to VARBINARY ostmt.defineColumnType(1, Types.VARBINARY); // Do a query to get the images named 'LESLIE' ResultSet rset = ostmt.executeQuery ("select GIFDATA from streamexample where NAME='LESLIE'"); // The data is not streamed here rset.next(); byte [] bytes = rset.getBytes(1);
If you use the defineColumnType() Oracle extension to redefine a CHAR, VARCHAR, or RAW column as a LONGVARCHAR or LONGVARBINARY, then you can get the column as a stream. The program will behave as if the column were actually of type LONG or LONG RAW. Note that there is not much point to this, because these columns are usually short.
If you try to get a CHAR, VARCHAR, or RAW column as a data stream without redefining the column type, the JDBC driver will return a Java InputStream, but no real streaming occurs. In the case of these datatypes, the JDBC driver fully fetches the data into an in-memory buffer during a call to the executeQuery() method or next() method. The getXXXStream() entry points return a stream that reads data from this buffer.
If your query selects multiple columns and one of the columns contains a data stream, then the contents of the columns following the stream column are not available until the stream has been read, and the stream column is no longer available once any following column is read. Any attempt to read a column beyond a streaming column closes the streaming column. See "Streaming Data Precautions" for more information.
Consider the following query:
ResultSet rset = stmt.executeQuery ("select DATECOL, LONGCOL, NUMBERCOL from TABLE"); while rset.next() { //get the date data java.sql.Date date = rset.getDate(1); // get the streaming data InputStream is = rset.getAsciiStream(2); // Open a file to store the gif data FileOutputStream file = new FileOutputStream ("ascii.dat"); // Loop, reading from the ascii stream and // write to the file int chunk; while ((chunk = is.read ()) != -1) file.write(chunk); // Close the file file.close(); //get the number column data int n = rset.getInt(3); }
The incoming data for each row has the following shape:
<a date><the characters of the long column><a number>
As you process each row of the iterator, you must complete any processing of the stream column before reading the number column.
An exception to this behavior is LOB data, which is also transferred between server and client as a Java stream. For more information on how the driver treats LOB data, see "Streaming LOBs and External Files".
There might be situations where you want to avoid reading a column that contains streaming data. If you do not want to read the data for the streaming column, then call the close() method of the stream object. This method discards the stream data and allows the driver to continue reading data for all the non-streaming columns that follow the stream. Even though you are intentionally discarding the stream, it is good programming practice to call the columns in SELECT-list order.
In the following example, the stream data in the LONG column is discarded and the data from only the DATE and NUMBER column is recovered:
ResultSet rset = stmt.executeQuery ("select DATECOL, LONGCOL, NUMBERCOL from TABLE"); while rset.next() { //get the date java.sql.Date date = rset.getDate(1); // access the stream data and discard it with close() InputStream is = rset.getAsciiStream(2); is.close(); // get the number column data int n = rset.getInt(3); }
The term large object (LOB) refers to a data item that is too large to be stored directly in a database table. Instead, a locator is stored in the database table and points to the location of the actual data. External files (binary files, or BFILEs) are managed similarly. The JDBC drivers can support these types through the use of streams:
LOBs and BFILEs behave differently from the other types of streaming data described in this chapter. The driver transfers data between server and client as a Java stream. However, unlike most Java streams, a locator representing the data is stored in the table. Thus, you can access the data at any time during the life of the connection.
When a query selects one or more CLOB or BLOB columns, the JDBC driver transfers to the client the data pointed to by the locator. The driver performs the transfer as a Java stream. To manipulate CLOB or BLOB data from JDBC, use methods in the Oracle extension classes oracle.sql.BLOB and oracle.sql.CLOB. These classes provide functionality such as reading from the CLOB or BLOB into an input stream, writing from an output stream into a CLOB or BLOB, determining the length of a CLOB or BLOB, and closing a CLOB or BLOB.
For a complete discussion of how to use streaming CLOB and BLOB data, see "Reading and Writing BLOB and CLOB Data".
An external file, or BFILE, is used to store a locator to a file outside the database, stored somewhere on the filesystem of the data server. The locator points to the actual location of the file.
When a query selects one or more BFILE columns, the JDBC driver transfers to the client the file pointed to by the locator. The transfer is performed in a Java stream. To manipulate BFILE data from JDBC, use methods in the Oracle extension class oracle.sql.BFILE. This class provides functionality such as reading from the BFILE into an input stream, writing from an output stream into a BFILE, determining the length of a BFILE, and closing a BFILE.
For a complete discussion of how to use streaming BFILE data, see "Reading BFILE Data".
You can discard the data from a stream at any time by calling the stream's close() method. You can also close and discard the stream by closing its result set or connection object. You can find more information about the close() method for data streams in "Bypassing Streaming Data Columns". For information on how to avoid closing a stream and discarding its data by accident, see "Streaming Data Precautions".
This section discusses several noteworthy and cautionary issues regarding the use of streams:
This section describes some of the precautions you must take to ensure that you do not accidentally discard or lose your stream data. The drivers automatically discard stream data if you perform any JDBC operation that communicates with the database, other than reading the current stream. Two common precautions are described:
To recover the data from a column containing a data stream, it is not enough to get the column; you must immediately process its contents. Otherwise, the contents will be discarded when you get the next column.
If your query selects multiple columns, the database sends each row as a set of bytes representing the columns in the SELECT order. If one of the columns contains stream data, the database sends the entire data stream before proceeding to the next column.
If you do not use the SELECT-list order to access data, then you can lose the stream data. That is, if you bypass the stream data column and access data in a column that follows it, the stream data will be lost. For example, if you try to access the data for the NUMBER column before reading the data from the stream data column, the JDBC driver first reads then discards the streaming data automatically. This can be very inefficient if the LONG column contains a large amount of data.
If you try to access the LONG column later in the program, the data will not be available and the driver will return a "Stream Closed" error.
The second point is illustrated in the following example:
ResultSet rset = stmt.executeQuery ("select DATECOL, LONGCOL, NUMBERCOL from TABLE"); while rset.next() { int n = rset.getInt(3); // This discards the streaming data InputStream is = rset.getAsciiStream(2); // Raises an error: stream closed. }
If you get the stream but do not use it before you get the NUMBER column, the stream still closes automatically:
ResultSet rset = stmt.executeQuery ("select DATECOL, LONGCOL, NUMBERCOL from TABLE"); while rset.next() { InputStream is = rset.getAsciiStream(2); // Get the stream int n = rset.getInt(3); // Discards streaming data and closes the stream } int c = is.read(); // c is -1: no more characters to read-stream closed
There is a limit on the maximum size of the array which can be bound using the PreparedStatement class setBytes() method, and on the size of the string which can be bound using the setString() method.
Above the limits, which depend on the version of the server you use, you should use setBinaryStream() or setCharacterStream() instead.
| Database Version | maximum setBytes() (equals maximum RAW size) | maximum setString() (equals maximum VARCHAR2 size) |
|---|---|---|
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Oracle8 and later |
2000 |
4000 |
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Oracle7 |
255 |
2000 |
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Note: This discussion applies to binds in SQL, not PL/SQL. If you use |
The 8.1.6 Oracle JDBC drivers may not raise an error if you exceed the limit when using setBytes() or setString(), but you may receive the following error:
ORA-17070: Data size bigger than max size for this type
Future versions of the Oracle drivers will raise an error if the length exceeds these limits.
If the JDBC driver encounters a column containing a data stream, row prefetching is set back to 1.
Row prefetching is an Oracle performance enhancement that allows multiple rows of data to be retrieved with each trip to the database. See "Oracle Row Prefetching".
This section describes how the Oracle JDBC drivers support the following kinds of stored procedures:
Oracle JDBC drivers support execution of PL/SQL stored procedures and anonymous blocks. They support both SQL92 escape syntax and Oracle PL/SQL block syntax. The following PL/SQL calls would work with any Oracle JDBC driver:
// SQL92 syntax CallableStatement cs1 = conn.prepareCall ( "{call proc (?,?)}" ) ; // stored proc CallableStatement cs2 = conn.prepareCall ( "{? = call func (?,?)}" ) ; // stored func // Oracle PL/SQL block syntax CallableStatement cs3 = conn.prepareCall ( "begin proc (?,?); end;" ) ; // stored proc CallableStatement cs4 = conn.prepareCall ( "begin ? := func(?,?); end;" ) ; // stored func
As an example of using Oracle syntax, here is a PL/SQL code snippet that creates a stored function. The PL/SQL function gets a character sequence and concatenates a suffix to it:
create or replace function foo (val1 char) return char as begin return val1 || 'suffix'; end;
Your invocation call in your JDBC program should look like:
Connection conn = DriverManager.getConnection ("jdbc:oracle:oci:@<hoststring>", "scott", "tiger"); CallableStatement cs = conn.prepareCall ("begin ? := foo(?); end;"); cs.registerOutParameter(1,Types.CHAR); cs.setString(2, "aa"); cs.executeUpdate(); String result = cs.getString(1);
You can use JDBC to invoke Java stored procedures through the SQL and PL/SQL engines. The syntax for calling Java stored procedures is the same as the syntax for calling PL/SQL stored procedures, presuming they have been properly "published" (that is, have had call specifications written to publish them to the Oracle data dictionary). See the Oracle9i Java Stored Procedures Developer's Guide for more information on writing, publishing, and using Java stored procedures.
To handle error conditions, the Oracle JDBC drivers throws SQL exceptions, producing instances of class java.sql.SQLException or a subclass. Errors can originate either in the JDBC driver or in the database (RDBMS) itself. Resulting messages describe the error and identify the method that threw the error. Additional run-time information can also be appended.
Basic exception-handling can include retrieving the error message, retrieving the error code, retrieving the SQL state, and printing the stack trace. The SQLException class includes functionality to retrieve all of this information, where available.
Errors originating in the JDBC driver are listed with their ORA numbers in Appendix B, "JDBC Error Messages".
Errors originating in the RDBMS are documented in the Oracle9i Error Messages reference.
You can retrieve basic error information with these SQLException methods:
getMessage()
For errors originating in the JDBC driver, this method returns the error message with no prefix. For errors originating in the RDBMS, it returns the error message prefixed with the corresponding ORA number.
getErrorCode()
For errors originating in either the JDBC driver or the RDBMS, this method returns the five-digit ORA number.
getSQLState()
For errors originating in the JDBC driver, this returns no useful information. For errors originating in the RDBMS, this method returns a five-digit code indicating the SQL state. Your code should be prepared to handle null data.
The following example prints output from a getMessage() call.
catch(SQLException e) { System.out.println("exception: " + e.getMessage()); }
This would print output such as the following for an error originating in the JDBC driver:
exception: Invalid column type
(There is no ORA number message prefix for errors originating in the JDBC driver, although you can get the ORA number with a getErrorCode() call.)
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Note: Error message text is available in alternative languages and character sets supported by Oracle. |
The SQLException class provides the following method for printing a stack trace.
printStackTrace()
This method prints the stack trace of the throwable object to the standard error stream. You can also specify a java.io.PrintStream object or java.io.PrintWriter object for output.
The following code fragment illustrates how you can catch SQL exceptions and print the stack trace.
try { <some code> }
catch(SQLException e) { e.printStackTrace (); }
To illustrate how the JDBC drivers handle errors, assume the following code uses an incorrect column index:
// Iterate through the result and print the employee names // of the code try { while (rset.next ()) System.out.println (rset.getString (5)); // incorrect column index } catch(SQLException e) { e.printStackTrace (); }
Assuming the column index is incorrect, executing the program would produce the following error text:
java.sql.SQLException: Invalid column index at oracle.jdbc.dbaccess.DBError.check_error(DBError.java:235) at oracle.jdbc.OracleStatement.prepare_for_new_get(OracleStatemen t.java:1560) at oracle.jdbc.OracleStatement.getStringValue(OracleStatement.jav a:1653) at oracle.jdbc.OracleResultSet.getString(OracleResultSet.java:175 ) at Employee.main(Employee.java:41)
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